Light emitting diodes and laser diodes are well known solid state electronic devices capable of generating light upon application of a sufficient voltage. Light emitting diodes and laser diodes may be generally referred to as light emitting devices (LEDs). Light emitting devices generally include a p-n junction formed in an epitaxial layer grown on a substrate such as sapphire, silicon, silicon carbide, gallium arsenide and the like. The wavelength distribution of the light generated by the LED depends on the material from which the p-n junction is fabricated and the structure of the thin epitaxial layers that include the active region of the device.
Typically, an LED includes a substrate, an n-type epitaxial region formed on the substrate and a p-type epitaxial region formed on the n-type epitaxial region (or vice-versa). In order to facilitate the application of a voltage to the device, an anode ohmic contact is formed on a p-type region of the device (typically, an exposed p-type epitaxial layer) and a cathode ohmic contact is formed on an n-type region of the device (such as the substrate or an exposed n-type epitaxial layer).
In order to use an LED in a circuit, it is desirable to package the LED to protect it from environmental harm and mechanical damage. An LED package also includes means, such as electrical leads, for electrically connecting the LED chip to an external circuit. In a typical package 70 illustrated in FIG. 1A, an LED 72 is mounted on a reflective cup 73 by means of a solder bond or epoxy. One or more wirebonds connect the ohmic contacts of the LED to leads 75A, 75B which may be attached to or integral with the reflective cup 73. The entire assembly is then encapsulated in a clear protective resin 74 which may be molded in the shape of a lens to collimate the light emitted from the LED chip 72.
In another conventional package 80 illustrated in FIG. 1B, a plurality of LED chips 82 are mounted onto a printed circuit board (PCB) carrier 83. One or more wirebond connections are made between ohmic contacts on the LEDs 82 and electrical traces 85A, 85B on the PCB 83. Each mounted LED 82 is then covered with a drop of clear resin 84 which may provide environmental and mechanical protection to the chip while also acting as a lens. The individual packaged LEDs 82 are then separated by sawing the PCB board 83 into small squares, each of which contains one or more LED chips 82.
One drawback to these methods of packaging an LED chip may be that the chips are mounted on the carriers one at a time. That is, each LED is individually mounted onto the PCB or a reflector cup. In addition, wirebond connections are typically made to each LED chip. These operations may be costly, time-consuming and may require substantial amounts of manual labor and/or specialized equipment. Problems with wirebond connections may also account for device failures in the field. Moreover, alignment and placement problems associated with mounting the LED chips may result in undesirable variations in optical characteristics of the resulting packaged chips.
One figure of merit for users of light emitting devices is cost per lumen, that is, the cost of obtaining a given level of light output. The high cost of conventional packaging techniques may be one factor that keeps the cost per lumen of solid state lighting relatively high. In addition, conventional packaging techniques may result in large, bulky packages that are unsuitable for certain miniaturized applications such as cellular telephone backlights. Conventional packaging techniques may also have poor thermal resistance characteristics that limit the power levels at which the LED chip may be driven and place constraints on system designers with respect to placement of LEDs.